Synthesis and characterisation of Sr₄Fe_3-xCr_xO_10-δ : stabilisation of n=3 Ruddlesden-Popper phases through Cr doping

Ruddlesden-Popper type compounds have the general formula, A _n+1M _nO _3n+1±x (typically A is a rare earth, alkaline earth, M is a transition metal), and are constructed of perovskite-type layers separated by rock salt type blocks. While n ​= ​1, 2 phases are typically straightforward to prepare, the synthesis of higher order (n ​> ​2) systems is difficult. In this paper we show that chromate (CrO ₄ ²⁻) doping can be exploited to stabilise new n ​= ​3 Ruddlesden-Popper systems, Sr ₄Fe _3-xCr _xO _10-δ: without chromate doping, a mixture of the n ​= ​2 phase Sr ₃Fe ₂O _7-x and perovskite-type SrFeO _3-x is obtained. This can be explained by the stabilisation of the central perovskite building block by chromate incorporation, similar to prior work on sulfate and carbonate doping in this system. The structure, and Fe/Cr oxidation states were evaluated by X-ray diffraction, ⁵⁷Fe Mössbauer spectroscopy and X-ray absorption spectroscopy supporting the incorporation of Cr as CrO ₄ ²⁻. In order to examine the potential of these new systems for use as a SOFC cathode material, conductivity studies were carried out, which showed semiconducting behaviour with slightly higher conductivities than the sulfate doped counterparts.